Method of producing nanophase Cu-Al2O3 composite power

ABSTRACT

The present invention relates to a method of producing nanophase Cu—Al 2 O 3  composite powder by means of 1) the producing precursor powders by centrifugal spray drying process using the water base solution, in which Cu-nitrate (Cu(NO 3 ) 2 3H 2 O) and Al-Nitrate (Al(NO 3 ) 3 9H 2 O) are solved to the point of final target composition (Cu-1 wt %/Al 2 O 3 ),2) the heat treatment process (desaltation process) at the 850° C. for 30 min in air atmosphere to remove the volatile components such as the moisture and NO 3  group in precursor powder and simultaneously to synthesize the nano CuO—Al 2 O 3  composite powders by the oxidation of corresponded metal components and 3) the reduction heat treatment of CuO at 200° C. for 30 min in reducing atmosphere to produce the final nanophase Cu—Al 2 O 3  composite powders with the size below 20 nm.

TECHNICAL FIELD

[0001] The present invention relates to a new method of produce theCu—Al₂O₃ composite powder with the particle size less than 20 nm bythermo-chemical process. Its technology comprised in (1) the producingprecursor powders by spray drying of water base solution made ofwater-soluble Cu and Al nitrates, (2) the heat treatment process(desalting process) in air atmosphere to remove the volatile componentssuch as the moisture and NO₃ group, and simultaneously to synthesize thenanophase CuO—Al₂O₃ composite powders and (3) the reduction process ofCuO to Cu to produce the nanophase Cu—Al₂O₃ composite powder.

BACKGROUND OF THE INVENTION

[0002] It is well known that if the alumina particles are extremely fineand homogenous in the dispersion-strengthened Cu matrix-Al₂O₃ dispersedcomposite, the material strengths at room temperature can be stillmaintained up to the near melting point of copper due to the excellentdispersion-strengthening effect. And the loss of matrix electricconductivity is not also significant because the total alumina volumefraction can be minimized in favor of the nano particle size.

[0003] Consequently, based on the aforementioned characteristics ofCu—Al₂O₃ composites, these materials are extensively used as the highstrength electric materials such as electrode for spot welding, leadwires, relay blades, and contact supports, and so on.

[0004] Especially, in the case of electrode materials used as theassembly for spot welder to join the body of the large vehicles, trainsor robots, it requires superior strength at a high temperature andarcwear resistance for electrical discharge.

[0005] The casting or conventional powder metallurgy (P/M) process arenot suitable to produce such materials, because these processes alwaysresult in the possibility of segregation of hard particles and thelimitation to produce or to distribute to ultra fine particles uniformlyinto metal matrix. Consequently these components should be manufacturedby the P/M process through the preparation of composite powdercontaining the nano alumina particles.

[0006] Conventionally the nanophase Cu—Al₂O₃ composite powder has beenproduced by internal oxidation process. This method consists offollowing steps: (1) gas atomizing process to produce the Cu/Al alloypowder, (2) heat treatment in air or oxygen contained atmosphere forinternal oxidation to produce CuO and Al₂O₃ composite powder, and (3)production of Cu—Al₂O₃ composite powder by means of the reducing CuO toCu in hydrogen atmosphere.

[0007] The particle size of Al₂O₃ in the composite powder produced bymeans of the aforementioned internal oxidation has been known to 50 μmin average. And oxygen during internal oxidation is diffused along thecoarse Cu grain boundary. It can result in a segregation of oxide phasein grain boundary, moreover such non-uniformed hard phase can affectnegatively the mechanical properties of the bulk materials produced byextrusion process.

SUMMARY OF THE INVENTION

[0008] Accordingly, in solving the aforementioned problems, thetechnical objective of the present invention lies in producing nanophaseCu—Al₂O₃ composite powder having homogenous distribution of oxideparticles.

[0009] Therefore the method of the present invention comprises asfollows:

[0010] 1. The process of preparing a water base solution made bydissolving the water-soluble salts of Cu-Nitrate (Cu(NO₃)₂3H₂O) andAl-Nitrate (Al(NO₃)₃9H₂O) for spray drying process.

[0011] 2. The spray drying of prepared solution for producing theprecursor powder consisted of molecular scale mixture of Cu, Al,NO₃—groups and moisture.

[0012] 3. The process of desalting heat treatment in air atmosphere toremove the volatile components such as the moisture and NO₃ andsimultaneously to synthesize the nanophase CuO—Al₂O₃ composite powders.

[0013] 4. The process of the heat treatment in hydrogen atmosphere toreduce the copper oxide to pure copper and finally to produce thenaonphase CuO₃—Al₂O₃ composite powders.

BRIEF DESCRIPTION OF DRAWING

[0014]FIG. 1 is a flowchart for producing nanophase Cu—Al₂O₃ compositepowder.

[0015]FIG. 2 is micrographs of the samples as per repective steps of theprocess: (a) spray dried powder, (b) desalted powder, (c) reducedpowder, (d) sintered bodies.

[0016]FIG. 3 shows the results of X-ray diffraction analysis of thepowders presented on the FIG. 2, i.e., the spray dried powder, desaltedpowder, and reduced powder.

[0017]FIG. 4 shows the results of X-ray diffraction analysis of reducedpowders, depending on the reduction times and temperatures.

[0018]FIG. 5 is (a) micrographs and (b) X-ray diffraction pattern of theAl₂O₃ particles extracted from the reduced powder after the desaltingheat treatment at 850° C. for 30 min.

[0019]FIG. 6 shows the results of the X-ray diffraction analysis ofAl₂O₃ phase within reduced powder, depending on time and temperature ofpreliminary desalting heat treatment of precursor powder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] The present invention is described by means of examples asfollows:

EXAMPLE 1

[0021] With the weight measurements of Cu-nitrate and Al-nitrate to thepoint of final target composition (Cu-1 wt % Al₂O₃), the solution wasprepared by dissolving the nitrates in distilled water. Then thesolution was fed on the rotational disk to form the fine droplet intothe hot chamber, and sprayed droplets are changed to precursor powder bydrying in the atmosphere of the hot air. The condition of spray drying,were set as follows: the temperature of intake air is about 240-260° C.,the temperature of exhaust air is 100-130° C., disk rotation speed liesin the range of 8,000-14,000 rpm, and the solution feed rate is 30-80ml/min.

[0022] Shape, size and structure of precursor powder are observed byscanning electron microscopy (SEM) and X-ray diffraction (XRD) methodsin FIG. 2(a) and FIG. 3, respectively.

[0023] As shown in the above figures the sizes of precursor powders arein the range of 20-80 μm, and in results of XRD analyses it hasamorphous state, it means that the precursor powder has homogeneouscohesion of respective components of Cu, Al and NO₃ at a molecularlevel.

[0024] Then the precursor powders are desalted by heat treatment in theair atmosphere at 850° C. for 30 min. XRD and SEM results afterdesalting process are shown in FIGS. 2(b) and 3 respectively. As shownin the XRD results the most of desalted powders were CuO, and thenon-existence of Al₂O₃ peaks was due to fact that the amount of thiscomponent was lower than background trace.

[0025] After that, the powder consisted of CuO and Al₂O₃ was reduced at150° C. for 1 hour, 200° C. for 30 minutes, and 200° C. for 1 hour inthe hydrogen atmosphere. The results of XRD analysis for reduced powdersare shown in FIG. 4 correspondingly.

[0026] As shown in FIG. 4, a the powder after reduction at 150° C. for 1hour was determined to be mixed state of CuO and Cu, which suggests thatCuO is not completely reduced yet. In the reduction at the 200° C. for30 min 1 hour, there was no CuO phase (FIG. 4), which implies completereduction. So, the optimal reduction conditions can be considered at200° C. for 30 minutes. FIGS. 2(c) and 3 were obtained from the reducedpowder produced through its optimal reduction conditions.

[0027] It was difficult to observe the particle size of copper directlyin the structure of reduced powder, because these particle sizes are toofine for observation. Consequently to observe the copper particles, thepower was compacted by normal way and sintered at 700° C. for 30 min andthen ached. FIG. 2(d) is the SEM microstructure of the sintered sampleobserved at the high magnification (×200,000). Here the particles of Cuare approximately 20 nm or less in diameter. As such, in considerationof the possibility of Cu particle growth during sintering, the size ofCu particles in the reduced powder would be less than that of sinteredstate.

[0028] For the purposes to observe the Al₂O₃ particles within thereduced powder, the base component of the reduced powder(i.e., Cu) wasdissolved in aqueous solution of nitric acid, and then the particles ofAl₂O₃ were extracted by filtering with 0.45 μm in mesh. To dissolve Cumatrix, the concentration of nitric acid in the aqueous solution was setto be 25 vol. %, and the dissolving time was required to 24 hours. TheSEM micrographs of extracted particles and its X-ray diffraction resultare shown in FIGS. 5(a) and 5(b), respectively, confirming that theAl₂O₃ were in gamma alumina phase (phaseR) with the size of 10-20 nm.

[0029] Consequently, as shown by the results of Example 1, the fineCu—Al₂O₃ composite powder with the particle size of 20 nm or less couldbe produced by the method of the present invention. Further, forobserving the behavior of Al₂O₃ formation, depending on desaltation timeand temperature, the experiments were carried out in Example 2 with saidvariables.

EXAMPLE 2

[0030] The processes for preparing the solutions, the spray-dryingprocess, and the reduction of oxide powder were carried out in the samemanner as Example 1. The powders were prepared with the variabledesaltation time and temperature. Then Cu was dissolved and extracted inthe same manner as Example 1. Extracted alumina oxide particles wereinvestigated by XRD method. The desaltation temperatures were 550, 750,and 850° C. respectively (850° C. in used in Example 1) and at thesetemperatures the treating time was 30 minutes and 2 hours. The resultsthereof are shown in FIG. 6.

[0031] As shown in FIG. 6, irrespective with the time, after thetreatment at the temperatures of 550° C. and 750° C., there was no gammaalumina phase. But at the temperature of 850° C. (as in Example 1), thegamma alumina oxide phase appeared in all cases, irrespective withtreating time. It leads to conclusion that the optimal temperature forthe formation of stable alumina particles is 850° C.

[0032] The table as below is a summary of the conditions and results ofExamples 1 and 2. As shown below, the optimal desaltation condition wasat 850° C. for 30 min. The Optimal reduction temperature was 200° C. forthe time of 30 min. At these conditions, the highest quality of Cu—Al₂O₃composite powder could be produced at the lowest energy. Table ofSynthesis Conditions and Results of Nanophase Cu/Al₂O₃ Composite PowderProcess Conditions Results Preparing the Cu-Nitrate + (Mix ratios of theSolutions Al-Nitrate + respective components water may vary depending onthe target composition) Conditions for Centrifugal Spray-drying ParticleSize: Spray-drying Rotational Speed of the 20-80 μm Disc: 8,000-14,000rpm Particle Form: Temperature of Intake Amorphous Air: 240° C.-260° C.Temperature of Exhaust Air: 100° C.-130° C. Feed Rate of Solutions:30-80 ml/min Desaltation 550° C. 30 minutes Synthesis of CuO withoutheat-treatment Al₂O₃ formation (in air) 2 hours Synthesis of CuO withoutAl₂O₃ formation 700° C. 30 minutes Synthesis of CuO without Al₂O₃formation 2 hours Synthesis of CuO without Al₂O₃ formation 850° C. 30minutes Synthesis of CuO and Al₂O₃ (Particle Size of Al₂O₃:approximately 20 nm) 2 hours Synthesis of CuO and Al₂O₃ (Particle Sizeof Al₂O₃: approximately 20 nm) Reduction 150° C. 1 hour IncompleteReduction (in H₂) 200° C. 1 hour Complete Reduction 30 minutes CompleteReduction (Particle Size of Cu: 20 nm or below)

[0033] The method of producing nanophase Cu—Al₂O₃ composite powder bymeans of the thermo-chemical method of the present invention has theeffect of enhancing the characteristics of the materials by producingthe nanophase Cu—Al₂O₃ composite powder with the size of 20 nm or below.Moreover the present invention has the effect of an economical benefitdue to the unlimited composition control, simplicity of process, and lowreaction temperature.

What is claimed is:
 1. A method of producing nanophase Cu—Al₂O₃composite powder, comprising: (i) preparing a water-base solution bycompletely dissolving Cu-nitrate and Al-nitrate in amounts correspondingto a final target composition for Cu—Al₂O₃ composite powder; (ii)preparing precursor powders having cohesion of Cu, Al, NO₃ and moistureat a molecular level by spray drying; (iii) producing a desalted powderby removing volatile components of NO₃ groups and partially reabsorbedmoisture by heat-treatment in air atmosphere, and simultaneouslyproducing nanophase CuO—Al₂O₃ composite powder by oxidation ofcorresponding metal components; and (iv) synthesizing nanophase Cu—Al₂O₃composite powder by means of reduction heat treatment of CuO within thedesalted powder in a reduced atmosphere.
 2. A method of producingnanophase Cu/Al₂O₃ composite powder according to claim 1 , wherein theconditions of said heat-treatment for desalting are set at a temperatureof at least 850° C. for at least 30 minutes.
 3. A method of producingnanophase Cu/Al₂O₃ composite powder according to claim 1 , wherein theconditions of said heat treatment for reducing CuO are set at atemperature of at least 200° C. for 30 at least minutes.